If so make distclean (which he did use) would be broken.
There are hosts which don't provide a way to do "make depend".

Any hint? Maybe something related to libtool?

Remove your CPPFLAGS and LDFLAGS. It's the -Wl,-R/usr/lib.

This gave me the right hint: I've removed the openSUSE RPM packages for
the OpenLDAP client 2.3 libs and now it works. It works with RE24
though. Is this because of different versions of the shared libs?

Ralf, any way to avoid that yast always wants to re-install the RPM
package openldap2-client? I've now added a file to /etc/ld.so.conf.d
which points to the self-compiled installation /opt/openldap-RE23/lib.
This used to work in former times. (Maybe your response off-list...)